Calibrate A Torque Wrench

The use of a torque wrench in the assembly of an engine, as well asother parts of a race car, is necessary for the correct tightening ofnuts and bolts. All threaded fasteners have a torque value. Materialsand designs are different, but the manufacturer has designated atightening torque for each.

Under-tightening does not provide proper thread loading. Over-tighteningcan stretch a fastener beyond its ability to recover. This weakening cancause the fastener to fail prematurely.

You should use a torque wrench to achieve the correct tightening torqueon any given fastener. But what if your torque wrench is inaccurate? Doyou have any idea if it is? If a torque wrench doesn't tell you thetruth, how do you properly assemble an engine?

The answer is that a torque wrench must be checked periodically.

Torque Lab

One business that does exactly that is Calibration Specialities ofIrving, Texas. The decision-maker there is Rick Heck. During a recenttour of the company, Heck showed me the lab where they test, repair, andadjust torque wrenches, among other things. There were severalworkstations and devices for testing from the largest to the smallesttorque wrench. There were hundreds of pigeon holes filled with smallparts for any brand of torque wrench. I watched as a wrench wasdisassembled. A few small parts were carefully changed and it went backto the test bench where it then passed.

I asked Heck about the tolerance to which a wrench should be held. "Atolerance of less than +/- 4 percent for the older wrenches isconsidered acceptable," he said. "Newer-style wrenches run to about +/-3 percent. These are the accepted norms."

What about the frequency at which wrenches are checked? "The airlinessend theirs in about once a year," Heck said. "Some racing enginebuilders check theirs as often as every 30 days. They'll do it moreoften if a wrench has been dropped or mishandled. We check it and adjustits accuracy if necessary. The turnaround time is usually a week or so."

Heck says a torque wrench should be handled like the precision tool itreally is. "Tossing them in the toolbox and digging them outoccasionally is not the way to keep them accurate," he says.

"In order to properly test, we have to keep our equipment checked to alevel of about one half of one percent of dead true," Heck says. "Theprofessionals at the airlines and professional race engine builders allhave their wrenches checked and adjusted as necessary."

MATERIALS LIST

1 piece of 1-in pipe, 8-ft long
2 pieces of 1-in square tubing,7-in long
1 piece of 1-in x 1-in flat bar or other material for a spacer
1 piece of 2-in angle, 5-in long
1 piece of 13/4-in tubing, 2-in long
1 5/8-in x 31/2-in bolt
3 5/8-in nuts
2 6202 ball bearings
1 package of J-B Weld
1 1/4-in or 1/2-in wide steel tape
1 gallon jug of water
25 pounds of balance weight